Coliform bacteria are a group of microorganisms commonly found in the environment and in the intestines of warm-blooded animals. Their presence in water is used as an indicator of potential pathogenic contamination.
Coliform bacteria are gram-negative, rod-shaped organisms that ferment lactose to produce acid and gas at 35-37 degrees Celsius within 48 hours. They are categorized into total coliforms, fecal coliforms, and Escherichia coli (E. coli), with each group providing increasing specificity about the source of contamination. Total coliforms include species found naturally in soil and vegetation, so their presence does not always indicate fecal contamination. Fecal coliforms, a subset of total coliforms, are more closely associated with the intestinal tracts of warm-blooded animals and are better indicators of sewage contamination. The U.S. EPA requires that public drinking water systems have no detectable total coliforms in 95 percent of monthly samples. Testing methods include the membrane filter technique, most probable number (MPN), and enzyme substrate tests such as Colilert. Elevated coliform counts in recreational waters can lead to beach closures and public health advisories.